The BBC pre-recorded the clash between Scotland's political leaders an hour before transmission and Labour's Chief of Staff, John McTernan, was tweeting journalists before the show had even started that it was very uncomfortable for Nicola Sturgeon.

The cause of his apparent glee was an admission from the SNP leader that she would introduce full fiscal autonomy for Scotland within a year if Westminster gave her the opportunity.

What this means is that the Scottish government would get control of nearly all taxes instead of relying on a block grant from Westminster for most of its revenue.

The problem for the SNP is that the plunging price of oil has left a black hole in the figures.

The Scottish government's White Paper on independence based its calculations for the finances of an independent Scotland on an oil price of $110 per barrel.

The current price of Brent Crude is not much more than half that at $59 per barrel.

Finances gap

According to the Institute for Fiscal Studies, the immediate shortfall in Scotland's finances would be £7.6bn. Labour want to know where that money would come from. Schools? Hospitals?

One Labour spinner described it as the biggest moment of the campaign so far.

Dave Watson, the head of campaigns at Unison, one of Scotland's biggest trade unions, dismissed Nicola Sturgeons position as bonkers.

So far, Labour's attempt to persuade SNP voters that they need to vote Labour to stop David Cameron winning a second term has had little effect.

The most recent YouGov opinion poll puts the Nationalists on 46 per cent, unchanged from the last survey in the middle of March. Labour are on 29 per cent, some 17 points behind.

It looks likely that the party will now dust down Better Together's campaign handbook from last year and attempt to bludgeon the SNP with an onslaught of negative stories about cuts.

Plenty of headlines

This is a strategy that will generate plenty of headlines and make Labour MPs feel good but it will ultimately ensure their defeat.

The basis of the SNP's lead rests in the apparent determination of those who voted Yes in September's referendum to follow that up with a vote for the SNP in May.

Professor John Curtice writes on the What Scotland Thinks website that unless voters can be persuaded to leave the independence debate to one side, it is difficult to see how Labour will be able manage to claw their way back to relative safety.

Instead of persuading voters to move on from the referendum, there is a real danger that Labour's campaign will simply reinforce those dividing lines.

To borrow an old cliche, Jim Murphy won a battle but he is losing the war.